(a) Orientation—The present-day marketplace for computer printing devices is extraordinarily competitive. Successful products for photo-type images no longer need only produce fine-looking prints, or operate at high throughput, or be inexpensive—but now must do all three.
Thus there is great commercial pressure to use multiple calorimetric levels to achieve smooth gradations for reproducing continuous-tone images, and to perform high-quality photo-like rendering—by relatively computation-intensive error diffusion rather than dither masking. (As will be seen, for smooth gradations it can be particularly important to employ multilevel processing in the rendition stage even if the final printing stage is only binary.) The market also demands high resolution, at least to the degree that the eye is able to discern this quality.
There is also great market pressure for very high processing speed, and this requirement is compounded when the apparatus is to print very large images—which of course entail processing extremely high quantities of data. At the same time, however, there is intense economic pressure to use only commercial off-the-shelf electronic processors, avoiding the cost of an ASIC to carry the error-diffusion burden.
Accordingly these market conditions present a formidable challenge to the limitations of the technology. For example, in the case of inkjet products associated with the present invention, for each printing-element array (printhead, or pen) it is desired to control printing of up to two dots in each pixel of a 24-by-24 dot/mm (600 dpi) grid, or up to one dot in each pixel of a 48-by-24 dot/mm (1200-by-600 dpi), in images of poster size.
(b) Previous efforts—Earlier generations of large-format products, e.g. of the Hewlett-Packard Company, approached these seemingly conflicting goals by using matrix-based binary printing, and performed the rendering and halftoning at a resolution lower than the final printing resolution—followed by some algorithm to artificially increase the resolution when printing the image.
Combining dithering and single-level halftoning imposed distinct limits to the image quality, and these are not acceptable in the present advanced generation of these devices. In addition, for lower-quality printmodes rendering was performed at lower resolution and then a method of pixel replication was used to print the lower-resolution images at a higher printing resolution—a very fast technique.
Image-quality defects, however, included patterning in area fills due to dithering, and objectionable artifacts arising from the replication. (Close inspection would reveal that 24 dot/mm pixels were identical across pixel groupings.) Naturally the method is inflexible in that the total number of drops printed in a certain area is the number of pixels set to one (“1”) in the rendered image, times the scaling factor.
Also known are various other devices and systems involving use of smaller pixel structures. The first above-mentioned Askeland document, for example, substitutes several extremely asymmetrical small pixels for each square standard-size pixel to produce very fine actual (not simulated) horizontal resolution). Other artisans have proposed insertion of a very localized finer-resolution data grid (e.g. for antialiasing). As far as the present inventors are aware, no such prior technique was for the purpose—or had the effect—of expanding a rendering resolution into a simulated final printing resolution throughout an image.
(c) Conclusion—Former configurations produced strikingly attractive printouts at modest cost and thus were successful within the level of development of the market then prevalent, but cannot satisfy the more-stringent current market demands outlined above. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.